We contributed to several rodent studies that investigated sources of variability in response to genetics and environmental agents. The NTP was considering including environmental enrichment materials (e.g., nesting materials, polycarbonate shelters) in rodent cages within its testing program. We designed a study and analyzed data to confirm that these materials will not affect toxicity endpoints in short-term rat and mouse studies. Some of the NTP Laboratory are investigating effects of environmental exposures on mammary gland development and mammary tumor development. Females of three strains of rats were examined at several time points during puberty to evaluate mammary gland development, including body weight, vaginal opening, hormone receptor binding within the gland, and quantitated mammary gland structure. There was considerable difference in the rate of development among the strains, suggesting that the strains had different windows during which exposure to carcinogens would be informative.